1. Field of the Invention
This invention relates to a hydraulic system comprising a hydraulic pump and a directional control valve and combined with a clamping unit for use in an injection molding machine, which clamping unit and hydraulic system comprise at least one hydraulic locking cylinder for generating the locking pressure for locking the injection mold, in which cylinder a piston defines cylinder chambers, which are adapted to communicate with each other through valve-controlled transfer passages of the piston, also comprise at least one hydraulic advance-retract cylinder, which contains a stationary piston and serves to close and open the injection mold and the movement of which is adapted to be controlled by means of a displacement-voltage converter and the directional control valve in conjunction with a control of the pressure on both sides of the stationary piston and in accordance with a fluid volume program with a superimposed automatic pressure control, and comprise at least one compensating cylinder, which is in open communication with the low-pressure chamber, wherein pressure sensors are provided in the communicating passages leading to the cylinder chambers of said advance-retract cylinder, which communicating passages communicate through branch passages to the high-pressure chamber and the low-pressure chamber, said branch passages are controlled by respective shut-off valves, and the pistons in the locking cylinder and the compensating cylinder are connected by associated piston rods to a movable mold carrier.
2. Description of the Prior Art
A hydraulic system of that kind is known from U.S. Pat. No. 4,846,664 and U.S application Ser. No. 07/450,258 and is operable to close an injection mold in that the mold-site cylinder chamber of the advance-retract cylinder is supplied via the control valve with a fluid under pressure, a superimposed automatic pressure control is effected by means of a pressure sensor. During the mold-closing movement, fluid is displaced from the rear cylinder chamber of the advance-retract cylinder and through a line provided with a pressure sensor for a superimposed automatic pressure control and through the directional control valve flows to the reservoir. Owing to the differential surfaces of the locking cylinder and of the compensating cylinder a small amount of pressure fluid is released during the mold-closing operation and that pressure fluid flows from the high-pressure chamber of the locking cylinder directly to the reservoir to effect a temperature compensation.